1. Field of the Invention
The invention relates to programmable integrated circuit devices, more particularly to the interconnect structure in a field programmable logic device.
2. Description of the Background Art
Field programmable gate arrays (FPGAs) include logic blocks connectable through a programmable interconnect structure. The interconnect structure typically provides for connecting each logic block to each other logic block. Early FPGAs accomplished this by providing short interconnect segments that could be joined to each other and to input and output terminals of the logic blocks at programmable interconnection points (PIPs). As these FPGAs become larger and more complex, the interconnect structure must also become both larger and more complex. In order to improve speed (performance), direct connections to adjacent logic blocks have been provided, and for transmitting a signal the distance of many logic blocks, longer lines have been provided. In order to save silicon area, less frequent PIPs have been provided. With fewer PIPs present, the routing is less flexible (for the same number of routing lines), but typically faster due to reduced loading. By removing only those PIPs which are least often used, routing flexibility can be minimally affected. Thus, there is a trade-off between performance, silicon area, number of routing lines, and routing flexibility.
Several U.S. patents show such structures for interconnecting logic blocks in FPGAS. Freeman in U.S. Reissue Pat. No. Re 34,363 describes the first FPGA interconnect structure, and includes short routing segments and flexible connections as well as global lines for signals such as clock signals. Carter in U.S. Pat. No. 4,642,487 shows the addition of direct connections between adjacent logic blocks to the interconnect structure of Freeman. These direct connections provide fast paths between adjacent logic blocks. Greene et al in U.S. Pat. No. 5,073,729 shows a segmented interconnect structure with routing lines of varied lengths. Kean in U.S. Pat. No. 5,469,003 shows a hierarchical interconnect structure having lines of a short length connectable at boundaries to lines of a longer length extending between the boundaries, and larger boundaries with lines of even longer length extending between those boundaries. Kean shows in particular lines the length of one logic block connecting each logic block to the next, lines the length of four logic blocks connectable to each logic block they pass, and lines the length of sixteen logic blocks connectable at the length-four boundaries to the length-four lines but not connectable directly to the logic blocks. In Kean's architecture, adjacent logic blocks in two different hierarchical blocks (i.e., on either side of the boundaries) connect to each other differently than adjacent logic blocks in the same hierarchical block.
Pierce et al in U.S. Pat. No. 5,581,199 shows a tile-based interconnect structure with lines of varying lengths in which each tile in a rectangular array may be identical to each other tile. In the Pierce et al architecture, an interconnect line is part of the output structure of a logic block. Output lines of more than one length extend past other logic block input lines to which the logic block output lines can be connected. All of the above-referenced patents are incorporated herein by reference, and can be reviewed for more understanding of prior art routing structures in FPGAs.
In the interconnect structures described by Freeman and Greene et al, each path is formed by traversing a series of programmably concatenated interconnect lines, i.e., a series of relatively short interconnect lines are programmably connected end to end to form a longer path. The relatively large number of programmable connections on a given signal path introduces delay into the signal path and therefore reduces the performance of the FPGA. Such interconnect structures are called "general interconnect".
The direct connections first described by Carter and included in the architecture of Kean provide fast paths between adjacent logic blocks, but in Carter's structure general interconnect must still be used to traverse the distance between any two blocks that are not adjacent. Therefore, circuits large enough or complex enough to require interconnecting signals between non-adjacent blocks (which frequently occur) must use the general interconnect to make these connections. For short paths, general interconnect is slower than direct interconnect, because general interconnect must be connected through several PIPs, or, if long lines are used, must be buffered to accommodate long or heavily loaded signals, introducing delay. Additionally, it is inefficient in terms of silicon area to use long lines for short paths that may be traversing only a few logic blocks, since the long lines can otherwise be used for longer paths. Further, since software that implements a logic design in an FPGA typically places interconnected logic in close proximity, structures that take advantage of this placement strategy will work well with the software, resulting in shorter compilation times for routing software and more efficient circuit implementations.
Interconnect lines called "quad lines" are included in the XC4000EX FPGAs from Xilinx, Inc., and described on pages 4-32 through 4-37 of the Xilinx 1996 Data Book entitled "The Programmable Logic Data Book", available from Xilinx, Inc., 2100 Logic Drive, San Jose, Calif. 95124, which are incorporated herein by reference. (Xilinx, Inc., owner of the copyright, has no objection to copying these and other pages referenced herein but otherwise reserves all copyright rights whatsoever.) However, since each quad line contacts every tile that it traverses, these lines have a large number of PIPs, each of which adds RC delay.
Pierce et al provides fast paths between both adjacent logic blocks and logic blocks several tiles apart. The output lines of the Pierce et al architecture can each drive the inputs of a limited set of other logic blocks. However, the possible destinations are limited to selected logic blocks, and the interconnect lines can only access certain specific inputs of the destination logic blocks.
In each of the prior art structures recited above, each interconnect line has programmable connections to the inputs of other logic blocks. However, in the structures of Freeman, Carter, and Pierce et al, a given logic block input can be driven from either horizontal interconnect lines, or vertical interconnect lines, but not both. An alternative approach is to separate the interconnect lines from the logic block inputs by way of a routing matrix, which gives each interconnect line more flexible access to the logic block inputs. Such an architecture is described in commonly assigned, co-pending U.S. application Ser. No. 08/618,445 entitled "FPGA Architecture With Repeatable Tiles Including Routing Matrices and Logic Matrices" by Tavana et al, which is referenced above and incorporated herein by reference. In the structure of Tavana et al, most interconnect lines entering the tile connect to a routing matrix within the tile, rather than directly to logic block inputs or outputs. Connections between pairs of interconnect lines and between interconnect lines and logic block inputs are made through lines called "tile interconnect lines" that do not leave the tile. The advantage of having an extra interconnect line in a path from the edge of a tile to the logic block in the tile is that the routing matrix is flexible but consumes a relatively small amount of silicon area. A combination of PIPs can allow access from any line entering the tile to any desired input of a destination logic block. Yet the total number of PIPs is smaller than in many other interconnect structures. The disadvantage is that getting on and off the tile interconnect lines inserts a certain amount of delay into the path for each tile traversed. This delay inhibits the fast propagation of signals through the FPGA. Tavana et al have therefore provided long lines connectable to every tile they pass and double-length lines that bypass the tile interconnect lines in one tile. These lines can be used for signals that are traversing one or more tiles without accessing the logic blocks in the traversed tiles.
Kean separates the interconnect lines from the logic block inputs using input multiplexer switches, which provide routing flexibility to the inputs.
Since the slowest signal path between logic blocks typically determines the performance of a circuit, it is advantageous to make the slowest path as fast as possible. One way to accomplish this is to design the interconnect structure such that there is a relatively uniform delay on all signal paths throughout an FPGA. In the above routing structures, a typical distribution of delays on signal paths shows a few signal paths with significantly greater delay than the average. These signal paths are typically those with large "RC trees", i.e., signal paths which traverse a resistor (such as an unbuffered PIP), then have a large capacitance on the destination side of the resistor. An interconnect structure with relatively uniform delay could be better realized if large capacitances on a signal path (e.g., longer interconnect lines) were predictably placed on the source side of the resistor, or as close as possible to the source end of the signal path.
High fanout signals have large capacitance and are often slower than low fanout signals. Prior art routing structures had high-fanout signal routing with relatively large RC delay. An interconnect structure should ideally provide high-fanout signal routing with a delay comparable to that of other signals.
It is therefore desirable to find an interconnect structure that allows: 1) uniformly fast propagation of signals, including high-fanout signals, throughout the FPGA; 2) implementation of localized circuits in non-adjacent as well as adjacent blocks using fast paths; 3) ease of use by software; 4) efficient implementation of commonly used logic functions; and 5) a high degree of routing flexibility per silicon area consumed.
Each interconnect line has certain characteristics that affect its speed and routing flexibility. One such characteristic affecting the speed of an interconnect line is whether or not it is buffered (driven by a buffer). One characteristic affecting the routing flexibility of an interconnect line is whether the line is unidirectional or bidirectional. When signal flow in a design implemented in an FPGA by a user is primarily in one direction (such as in a datapath where data flows primarily from one side of the FPGA to the other side), unidirectional lines in the "wrong" direction cannot be used effectively. Therefore, bidirectional interconnect lines are more flexible than unidirectional lines, since they can be used to route signals in whichever direction is most needed. However, forming a bidirectional interconnect line requires much more silicon area than forming a unidirectional interconnect line. Further, bidirectional lines are typically slower than unidirectional lines, because of increased loading and additional logic required in the signal path to implement the bidirectional capability. Therefore, there are drawbacks as well as advantages to having bidirectional capability on interconnect lines. It is desirable to provide bidirectional interconnect capability while limiting the impact on design performance and the amount of silicon area required.